Short answers, pulled from the story.
Planet Nine is a hypothetical Solar System planet with an estimated mass of 4.4 to 10 times that of Earth. The hypothesis gained significant traction in early 2016 when Konstantin Batygin and Michael Brown described how similar orbits of six extreme trans-Neptunian objects could be explained by its existence.
Planet Nine has a semi-major axis estimated at roughly 700 astronomical units or between 300 and 520 AU depending on reanalysis data. It takes approximately 10,000 to 20,000 years to complete one full orbit around the Sun.
Astronomers discovered Neptune in 1846 after noticing irregularities in Uranus's orbit. This discovery sparked speculation that another planet might exist beyond its orbit, leading Percival Lowell to begin an extensive search for this hypothetical trans-Neptunian planet in 1906.
Researchers point to the clustering of six extreme trans-Neptunian objects with perihelia opposite to each other as evidence for Planet Nine. Simulations show that swarms of scattered disk objects were sculpted into collinear groups by a massive distant planet with an eccentricity of approximately 0.6.
The Outer Solar System Origins Survey documented over 800 trans-Neptunian objects without finding evidence for clustering. Critics argue that gravitational occultation from Neptune during its outward migration explains these extreme orbits rather than a ninth planet.